A fair protocol for signing contracts

Michael Ben-Or, Oded Goldreich, Silvio Micali, Ronald L. Rivest

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

22 Scopus citations


Assume that two parties, A and B, want to sign a contract over a communication network, i.e. they want to exchange their “commitments“ to the contract. We consider a contract signing protocol to be fair if, at any stage in its execution, the following hold: the conditional probability that party A obtains B's signature to the contract given that B has obtained A's signature to the contract, is close to 1. (Symmetrically, when switching the roles of A and B). Contract signing protocols cannot be fair without relying on a trusted third party. We present a fair, cryptographic protocol for signing contracts that makes use of the weakest possible form of a trusted third party (judge). If both A and B are honest, the judge will never be called upon. Otherwise, the judge rules by performing a simple computation, without referring to previous verdicts. Thus, no bookkeeping is required from the judge. Our protocol is fair even if A and B have very different computing powers. Its fairness is proved under the very general cryptographic assumption that functions that are one-way in a weak sense exist. Our protocol is also optimal with respect to the number of messages exchanged.

Original languageAmerican English
Title of host publicationAutomata, Languages and Programming - 12th Colloquium
EditorsWilfried Brauer
PublisherSpringer Verlag
Number of pages10
ISBN (Print)9783540156505
StatePublished - 1985
Event12th International Colloquium on Automata, Languages and Programming, ALP 1985 - Nafplion, Greece
Duration: 15 Jul 198519 Jul 1985

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume194 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference12th International Colloquium on Automata, Languages and Programming, ALP 1985

Bibliographical note

Publisher Copyright:
© 1985, Springer-Verlag.


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